Convert calendar_dates to schedule exceptions on import into editor #80
Description
Currently (and since the earliest versions of gtfs-editor), when importing a feed into the editor the calendar_dates.txt table is entirely disregarded. Schedule exceptions must be generated and managed from scratch within the feed editor. This is due in part to the complex combinations of calendars.txt and calendar_dates.txt that can be found in GTFS feeds. One notable example, a case which is also described in the reference documentation, is the TriMet feed, which explicitly defines all dates of operation in calendar_dates.txt.
While there is no support currently for moving from calendar_dates.txt to editor-defined schedule exceptions, there may be some simple rules we could introduce to simply convert single or paired dates entries into the SWAP exception type.